When a roadbed is to be constructed using hot mix asphalt (HMA), the HMA typically is prepared off-site and transferred to the construction site by a transport machine such as, for example, a dump truck. The HMA, which typically is maintained at a temperature between 300 and 400 degrees Fahrenheit, often is loaded onto the dump truck using an elevator, which conveys the HMA to a spout from which it falls into the truck bed. At the construction site, the HMA can be transferred to a paving machine, which deposits the HMA on the roadbed.
If the HMA is to be temporarily stored at the construction site, the HMA can be transferred from the dump truck to a material transport vehicle (MTV). As the term is used herein, a material transfer vehicle (MTV) refers to any type of machine that can be used to transfer HMA to a paving machine. Exemplary MTVs include remixing transfer vehicles (e.g., “self-propelled” transfer machines that include their own drive system), windrow pickup machines (e.g., “towed” transfer machines that do not include their own drive system), components of paving machines that transfer HMA into the paving machine, and the like.
During such a transfer, the HMA exits a chute at the back of the truck and, in some cases, is transferred into a hopper on the MTV. A windrow elevator conveys the HMA from the hopper either directly into the paving machine or into another hopper, where it is temporarily stored until the paving machine is ready to use it. Alternatively, the dump truck may dump the HMA onto the roadbed, while it moves along the roadbed, thereby creating a windrow. In this case, the windrow is picked up from the roadbed (e.g., by a windrow pickup machine) and transferred to the paving machine.
When the HMA is loaded into truck beds, hoppers, and other containers, the HMA segregates. Additionally, segregation occurs when HMA is unloaded onto a roadbed to create a windrow. Larger, heavier pieces of HMA tend to roll to the outside (e.g., away from the peak) of the resulting piles or windrow (the process known as “segregation”). Moreover, during transportation and temporary storage, the load of HMA often develops temperature variations because the material toward the outside of the container tends to cool faster than the material toward the middle of the container. The amount of segregation and temperature variation in HMA has a negative impact on the roadbed's durability. Thus, HMA typically is desegregated (i.e., re-mixed) prior to its roadbed application. A number of devices such as the examples listed below have been developed to perform the desegregation.
U.S. Pat. No. 5,035,534, to Brock and Smith, for an Apparatus for Transferring an Asphalt-Aggregate Mixture, includes transversely disposed variable pitch screw augers on a single shaft mounted in the bottom of storage hopper. The first screw auger is located on one side of a mid portion of the shaft and the second is located on the other side of a mid portion of the shaft. Each of the screw augers has flights of a first pitch at the outer end of the shaft and flights of a second pitch set inwardly. This arrangement is described as allowing the HMA at the center portion of stream to combine with material transported inwardly to center. The description states that the variable pitches allow different sizes of HMA pieces to be mixed and explains that the mixing occurs due to and in the flights of the pitched augers. The material is mixed in the flights and moved toward the center where it falls through the material outlet underneath a cover.
U.S. Pat. No. 5,405,214, to Campbell, for a Paving Machine Incorporating Automatic Feeder Control Gates, uses a first gate mechanism and a second gate mechanism. Raising one and lowering the other shifts discharged materials to one side; raising and lowering the opposite shifts the material to the other allowing management of the flow along both a vertical and horizontal plane. Some mixing also occurs as a result.
U.S. Pat. No. 5,553,968, to Campbell, for a Method and Apparatus for Conveying and Desegregating Aggregate, uses a different approach. Rather than using adjustable gates to move outer materials to the center for remixing, this one uses a notch in the floor of a first drag slat conveyor. The notch is in the center of the first conveyor floor and positioned near the end of the first conveyance path. The small pieces fall through the notch on to a second conveyor, the large pieces fall later thereby re-orienting the small and large pieces along the direction of the conveyor, rather than outside to inside.
U.S. Pat. No. 5,642,961, to Campbell, for a Method for Conveying and Desegregating Aggregate, is similar to the U.S. Pat. No. 5,553,968 in that it reorients laterally segregated aggregate on a conveyor to a longitudinally segregated aggregate. The aggregate is then desegregated as it comes into contact with and travels along a second conveyor. This is achieved by discharging smaller pieces onto the second conveyor before the larger.
In U.S. Pat. No. 6,007,272, to Macku and Trygg, for an Asphalt Paver with Remixing Conveyor System, describes a paver, which includes a hopper conveyor to move HMA from hopper to screed, as having at least one pair of spaced apart axially rotatable augers. These augers are disposed in the direction of travel. Each auger has a “tapered peripheral diameter” defining a space therebetween as a “remixing zone.” U.S. Pat. No. 6,099,205, to Macku and Trygg, for an Asphalt Paver with Remixing Conveyer System, describes a similar apparatus.
U.S. Pat. No. 6,481,922, to Boyd, for an Apparatus and Method for Re-mixing Segregated Material, describes a device wherein the outer portions of a moving HMA stream are moved toward the center via augers. Here, the outer portions of the HMA stream drops through a material outlet with the inner portions of the stream. This patent terms the uncovered center portion of the augers, which is positioned directly over the material outlet, a mixing zone, explaining that this is the area in which mixing of the small and large pieces of HMA occurs.
U.S. Pat. No. 7,160,056, to Hoffman and Swearingen, for a Material Transfer Vehicle for Use in Asphalt Paving, describes a material transfer vehicle having a surge bin for storing HMA prior to paving. The surge bin includes counter-handed remixing augers that tend to move the HMA pieces to the center of the bin for remixing. One outlet is provided for releasing the HMA from the surge bin onto a transfer conveyor.
U.S. Pat. No. 7,785,033, to Boyd, for an Apparatus and Method for Fragmenting and Re-Mixing Agglomerated Pieces of Rubberized Asphalt Material, describes a housing having an auger and tine assembly that rotates within the housing. The assembly includes counter-handed auger sections mounted on a shaft, with a tine section mounted on the shaft between the auger sections. The re-mixed material exits the housing through a single opening disposed below the tine section.
The prior art concepts generally do not adequately accomplish the objectives of remixing the HMA to achieve significantly uniform distribution of large and small particulate HMA. For example, even where the distribution's level of uniformity is improved somewhat, the utility of the achievement is mitigated by the fact that the re-mixed HMA often is distributed in a single stream. As the stream is off-loaded, it forms a pile, and a number of large pieces of HMA will tend to roll to the edges of the pile, thereby re-segregating the HMA to a certain degree, as well as diminishing the uniformity of a roadbed profile of delivered HMA. Further, the level of HMA remixing tends to be inadequate with conventional systems and can often be better characterized as concentrating the stream of HMA pieces of all sizes rather than actually remixing it.
It is a first objective of the invention to improve the level of remixing of HMA prior to its delivery to a roadbed.
It is a second objective of the invention to provide a desegregation apparatus that accomplishes the first objective.
It is a third objective of the invention to provide an output stream of HMA that has a flatter profile.
It is a fourth objective to provide means to better mix both small and large pieces of HMA prior to delivery to the roadbed, thereby remixing the stream of HMA rather than simply concentrating the stream into a more narrow flow pattern.